Did a fresh install of libreelec on RPi 4 and I can confirm that 3D output on HDMI to tv doesn't work. It works fine on RPi 3. Also there seems to be a refreshing and v-sync issue on RPi 4 even without 3D. Just hopping that 3D playback won't be abandoned as it's one of the main reasons why I bought Raspberry Pi in the first place. It works fine on RPi 3, but the new one supports gigabit ethernet and 5GHz wifi which makes it ideal for home media center.